User Site
To make a main website for yourself, name is <user>.github.io.
Organization Site (A Website for a Group)
the name is like: <organization>.github.io
Project Site (A Website for a Specific Project)
http(s)://<user>.github.io/<repository> or http(s)://<organization>.github.io/<repository>。
For Personal or Organization Sites:
Your site must be in the master/main branch. It will publish automatically from this branch.
You cannot change this.
For Project Sites:
The default is the gh-pages branch.
but you can change it to master or a /docs folder.
https://lineuman.github.io/blog/
https://lineuman.github.io/mainpage/